Through hole vs. Surface mount solder fatigue

Why do through hole (TH) components take so much longer to fail than surface mount technology (SMT)?

Solder will fail due to fatigue if there is a cyclical shear load on the joint. The cyclic load in electronics comes from the coefficient of thermal expansion (CTE) mismatch between various materials. Once a crack initiates it will propagate along a path until reaching the other side of the joint. 

In a through hole part the CTE mismatch is between the board and the component lead. The CTE in the Z (out of plane) direction of the board will stretch the Plated Through Hole (PTH) and since the lead material has a different CTE the solder has to accommodate the difference. The crack path length is equal to the board thickness. The solder joint thickness is the difference between the outer radius of the lead and the inner radius of the PTH.

For SMT components the CTE mismatch is between the board and the component itself. For components with leads we can add the lead CTE and flexibility to the calculations for the total shear load on the solder joint. The solder joint thickness is usually small. Once a crack initiates it will follow a path that is the length of the pad for lead-less components or the lead foot length for components with leads.

The smaller CTE mismatch, longer crack path and thicker solder joints in through hole components can make them last longer than equivalent surface mount components. The clear advantages of using SMT components far outweigh the disadvantages but don't forget to account for the possibility of reduced reliability.

Copyright Gil Sharon July 8, 2015  . All rights reserved.